I frequently see older vets doing physical therapy at the base gym, recovering from knee or hip replacement. They're always fat and obviously haven't been in shape since they left active duty. Their joints failed after years of carrying too much weight all day, every day.
I can remember seeing just one person in really good shape who'd had joint surgery. She was a serious, 50+ year old competitive body builder. She'd had a torn rotator cuff, caused by bone spurs in her shoulder, not by the stress of exercise.